As is known in the art, thermal conduits are used in many applications. One application is in heat spreaders, sometimes also referred to as heat transfer interfaces in extracting heat from a heat source and transporting the exported heat to a heat sink. Further, in order to make a robust, high reliability head spreader, the coefficient of thermal expansion (CTE) between the heat spreader and the heat source should be closely matched and the CTE between the thermal spreader and the heat sink should be closely matched. In electrical applications, for example, these additional interfaces can result in degraded thermal performance and increased electrical parasitics thereby increasing electrical loss, reducing efficiency and/or adding cost and complexity to the system design.
Current method for heat spreader design has been to add intermediate heat spreader transitions between low CTE and typically poorer thermal conductivity materials to high thermally conductive and typically high thermal expansion materials such as copper as the CTE mismatch interface between the high conductivity metals and the low thermally conductive material semiconductor is, in many applications, too great to reliably make a direct transition. This approach adds design complexity, cost, and performance loss due to the additional thermal and electrical interfaces.
As is known in the art, Thermal Pyrolytic Graphite (TPG) material exhibits very anisotropic thermal conductivity such that, within the basal plane, the thermal conductivity can be ˜1600 W/m-° K (4× of copper) and perpendicular to the basal plane is ˜10 W/m-° K ( 1/40 of copper). More particularly, graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion. Still more particularly, graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. A sheet of pyrolytic graphite may be described as having three directional axes; an a-axis and a b-axis which are parallel to the surface of deposition of the basal planes and perpendicular to each other, and a c-axis of which is perpendicular to both the a-axis and the b-axis and to the basal planes. The thermal properties of pyrolytic graphite are strongly affected by its structural anisotropy. Pyrolytic graphite acts as an excellent heat insulator in the c-axis direction (which is along the direction of deposition of the graphite; perpendicular to the plane of the surface upon which the graphite is being deposited) and as a relatively good heat conductor in the planes containing the a-axis and the b-axes.
It should be understood that the term “thermal pyrolytic graphite” (“TPG”) may be used interchangeably with “highly oriented pyrolytic graphite” (“HOPG”), or compression annealed pyrolytic graphite (“CAPG”).
It is also known in the art that TPG materials have been used as anisotropic thermal conduits in planar beat spreaders as shown in FIG. 1; the basal plane being represented by dotted lines.